They shall determine the steroid-binding activity of milk, particularly with respect to progesterone, cortisol, 178 - estradiol, and testosterone. Raw milk from women, cows, and rats will be obtained at early and late stages of lactation. The effect of administered prolactin and cortisol on the levels of the steroid-binding activity of milk from lactating lactating rats will be studied also. The whey proteins will be subjected to various fractionation procedures so that the protein components of milk which are responsible for specific steroid-binding activity may be purified and partially characterized by physicochemical methods.